In a small brick building on the edge of a cornfield in Romeoville, three scientists sit before a bank of glowing computer monitors and attempt to answer a common query: What will the weather be like this week?

It's a deceptively simple question — one that would have been absurd about 40 years ago when meteorologists whipped thermometers in circles to measure humidity and traced storms on paper using grease pencils.

But unlike their predecessors, these scientists have modern-day meteorology at their fingertips. Satellite images show clouds churning across the globe; radar beams reflect nearby wind, rain or hail; and supercomputers crunch all that and more into stunningly accurate weather predictions at 70 trillion mathematical calculations per second.

Meteorologist Paul Merzlock, who has been predicting the weather for northern Illinois since the 1980s, grasped for an adequate analogy.

"It's like going from pencil sharpening to 'Star Wars,'" he said. "It's like leaping from the Stone Age to the Space Age."

And at no time of the year is the importance of this leap more evident than in the so-called "convective season," otherwise known as spring.

From about April to June, as cold air barrels down the Great Plains and collides with warmer, humid air rising out of the Gulf of Mexico, explosive thunderstorms with tornado-producing potential often rip through the Chicago area.

How much time people have to duck under umbrellas or into basement shelters comes down to meteorologists like Merzlock who employ a worldwide mesh of weather-sensing machinery.

"These thunderstorms look pretty impressive," said Merzlock, 54, on a recent Friday in Romeoville as he pointed to a crimson streak blowing toward Rockford on his computer screen.

"The really dark reds on this radar image indicate really high energy returns, which is probably hail," said Merzlock, whose well-tanned, creased face hints at a life spent in the weather he tracks. "We'll be issuing a tornado watch here in a couple of minutes."

The developing storm did not surprise the staff of the Romeoville office, which monitors a 23-county area around Chicago. They had been predicting bad weather all week, starting the previous Monday with satellite images of rain clouds arcing over the Pacific Northwest.

From there, hundreds of electronic sensors around the country transmitted atmospheric measurements, like temperature and pressure, to supercomputers in Washington, D.C. Applying the laws of physics, those high-powered computers then sent data and images to the Romeoville meteorologists, projecting how the storm front would develop in the coming days.

Using about a dozen models generated from computers in the United States, the United Kingdom, Canada and Europe, the scientists finally pieced together their seven-day weather forecast.

Dr. Louis Uccellini, director of the National Centers for Environmental Prediction, attempted to put the current computing power into perspective: If everyone in the United States were doing a calculation per second, it would take them about 2 1/2 days to do the amount of work these computers can do in one second, he said.

"It is one of the top intellectual achievements of the 20th century," Uccellini said. "Just 45 years ago, there was no credibility for specific weather forecasts of extreme events beyond 24 hours."

In the 1960s, general weather forecasting stretched only about two days into the future. At the time, a detailed seven-day weather forecast would have been considered "foolhardy," Merzlock said.

Weather forecasting has progressed at about a day a decade, Uccellini said. Today's seven-day forecasts are about as accurate as five-day forecasts in the mid-1990s, he said.

Yet, despite all the advances, geographically small events — like a tornado — can still be hard to foretell.

It's a fact that can keep the meteorologists in Romeoville up at night.

On that date, an unexpected tornado emerged from a severe thunderstorm, killing 29 people, injuring at least 300 and causing more than $200 million in property damage in about 15 minutes.

At the time, forecasters lacked Doppler radar, which would likely have revealed the rotating columns of air that often lead to a tornado. Instead, warnings of that impending storm were issued too late. Mark Ratzer, who also forecasts weather for Chicago, called the destruction after a catastrophic weather event "sobering."

"To actually have a fatal event, you remember that," said Ratzer, who witnessed the aftermath of the 2004 Utica, Ill., tornado. "You think about what you could have done."

In the future, meteorologists hope that by increasing radar and computing power, they will be able to predict tornadoes much like a rain forecast. In January, the Romeoville meteorologists plan to install a new radar system that will be able to visualize the size and shape of particles in a storm, giving scientists an even better grasp of the storm's severity.

Until that time, however, meteorologists issue the standard tornado watch, review the incoming data and wait for the winds to turn.